<p>Ferronickel slag represents the most prevalent byproduct in the production of nickel–iron alloys. The utilization of this slag to produce mineral wool not only facilitates the comprehensive recycling of the byproduct, but also capitalizes on the waste heat from the nickel–iron production process. The overall utilization rate of energy has been improved. To achieve slag with suitable properties for mineral wool production, it is crucial to control its physical and chemical characteristics, such as crystallization behavior and flow dynamics. Confocal Laser Scanning Microscope (CLSM) was employed to observe the in situ isothermal crystallization process of B₂O₃ treated ferronickel slag at various temperatures in our research, as well as its continuous crystallization behavior under different cooling rates. Additionally, a high-temperature rotational viscometer was used to examine the relationship between cooling rate and viscosity. The results indicate that the isothermal crystallization process is fastest at 1200&#xa0;℃. When the cooling rate is greater than 30&#xa0;℃/s, the continuous crystallization of slag will result in an amorphous structure. The transformation temperature of the ferronickel slag was found to be approximately 1330&#xa0;℃. Furthermore, the increase of the cooling rate leads to a reduction in slag viscosity. The transition temperature decreases as the cooling rate rises. Finally, fiber testing was conducted on the mineral wool products. The results indicated that the mineral wool containing boron exhibited improved forming performance and higher tensile strength. This research provides valuable theoretical insights and empirical data to support the use of solid waste in the production of mineral wool.</p> Graphical Abstract <p></p>